Rapid spread of radio equipment typified by cellular phones increases the demand for small and light resonators and filters including a combination of the resonators. Hitherto, mainly, dielectric filters and surface acoustic wave (SAW) filters have been used. Nowadays, filters using piezoelectric thin film resonators are receiving attention because the piezoelectric thin film resonators are elements that achieve low-loss especially in a high frequency range, high power handling capability, and satisfactory electrostatic discharge (ESD) characteristics and can be implemented in monolithic form.
A film bulk acoustic resonator (FBAR) type resonator is known as one of such piezoelectric thin film resonators. This resonator includes a laminate that includes, as main components, a lower electrode, a piezoelectric film, and an upper electrode on a substrate. An air gap (a via hole or a cavity) is formed under a portion (a membrane region) of the lower electrode that opposes the upper electrode.
When a high frequency electrical signal is applied between the upper electrode and the lower electrode, an acoustic wave is excited in the piezoelectric film sandwiched between the upper electrode and the lower electrode by the inverse piezoelectric effect. Distortion due to the acoustic wave is converted to an electrical signal by the piezoelectric effect. Since the acoustic wave is totally reflected at a surface of each of the upper electrode and the lower electrode that is brought into contact with air, the acoustic wave is converted to a thickness extensional vibration wave having a major displacement in the thickness direction. In this structure, resonance occurs at a frequency at which a total film thickness H of a thin film structure is an integral multiple (n times) of half of the wavelength of the acoustic wave, where the thin film structure includes, as main components, the upper electrode, the piezoelectric film, and the lower electrode formed above the air gap. Assuming that V is the propagation velocity of the acoustic wave determined by the material, a resonance frequency F is:F=nV/2H. A resonator having desired frequency characteristics can be fabricated by controlling the resonance frequency via the film thickness H through the use of the resonance. Moreover, a filter can be fabricated by connecting a plurality of resonators.
An electrode may be made of aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), titanium (Ti), or the like, or a laminated material composed of these materials.
Silicon, glass, gallium arsenide (GaAs), or the like may be used as a substrate. An air gap is formed by, for example, etching (wet etching or dry etching) of an Si substrate used as an element substrate from the back surface or wet etching of a sacrificial layer provided on a surface of an Si substrate. Hereinafter, a via hole refers to a hole that extends from the back surface of a substrate to the front surface of the substrate, and a cavity refers to an air gap that exists directly under a lower electrode in the neighborhood of a surface of a substrate. Known piezoelectric thin film resonators may be classified into via hole type piezoelectric thin film resonators and cavity type piezoelectric thin film resonators.
FIG. 25 is a cross sectional view illustrating the structure of an exemplary via hole type piezoelectric thin film resonator disclosed in, for example, K. Nakamura, et al, “ZnO/SiO2-Diaphragm composite resonator on a silicon wafer”, Electronics Letters, Vol. 17, No. 14, P.507-P.509, 1981. In this structure, Au—Cr, zinc oxide (ZnO), and aluminum (Al) are respectively formed as a lower electrode, a piezoelectric film, and an upper electrode on an (100) Si substrate that includes a thermal oxide film (SiO2). A via hole 106 is formed by performing anisotropic etching on the Si substrate 101 from the back surface using a KOH aqueous solution or an EDP aqueous solution (ethylenediamine+pyrocatechol+water).
On the other hand, in a cavity type piezoelectric thin film resonator, an upper electrode, a piezoelectric film, and a lower electrode are formed as main components on a sacrificial layer, and finally, a cavity is formed by removing the sacrificial layer by etching. FIG. 26 illustrates a cross sectional view of a cavity type piezoelectric thin film resonator disclosed in Japanese Examined Patent Application Publication No. 1994-40611. In this example, an island-shaped sacrificial layer pattern of ZnO is prepared as a sacrificial layer, a structure that includes a dielectric film 205, an upper electrode 202, a piezoelectric film 204, a lower electrode, and a dielectric film 203 is prepared on the sacrificial layer, and a cavity 206 (an air bridge structure) is formed by removing the sacrificial layer using acid.
Moreover, FIG. 27 illustrates a cross sectional view of a cavity type piezoelectric thin film resonator disclosed in Japanese Laid-open Patent Publication No. 2000-69594. This structure represents a piezoelectric thin film resonator in which a depression 306 is provided in a substrate surface below a region where an upper electrode 302, a piezoelectric film 304, and a lower electrode 303 overlap with each other. In the piezoelectric thin film resonator illustrated in FIG. 27, after a sacrificial layer is deposited on the depression 306 formed in advance to flatten the substrate surface, and the upper electrode 302, the piezoelectric film 304, and the lower electrode 303 are formed. Finally, a cavity is formed by removing the sacrificial layer by etching.
As a piezoelectric film, aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3), or the like may be used. In practice, in many cases, AlN is used in terms of acoustic velocity, temperature characteristics, the Q-value, and easiness of the film deposition technique. In particular, one of important factors determining the resonance characteristics is forming a highly crystalline AlN film that is c-axis (perpendicular to a lower electrode surface) oriented. This factor directly affects the coupling coefficient and the Q-value. On the other hand, when a highly crystalline AlN film that is c-axis oriented is deposited, a high energy needs to be applied. For example, in metal organic chemical vapor deposition (MOCVD), a substrate needs to be heated at a temperature of at least 1000° C. Even in plasma enhanced chemical vapor deposition (PECVD), in addition to electrical power for plasma, a substrate needs to be heated at a temperature of at least 400° C. It is known that, even when a sputtering technique is used, the temperature of a substrate is increased by sputtering of an insulator film. Thus, in general, an AlN film has a strong membrane stress.
Japanese Laid-open Patent Publication No. 2005-347898 discloses that a cavity of the order of several micrometers high can be formed even with a sacrificial layer of the order of several hundred nanometers thick by controlling the stresses of upper and lower electrodes and AlN to inflate a region (hereinafter called a membrane region) where the upper and lower electrodes and AlN overlap with each other on an air gap.
FIG. 28A is a plan view for illustrating the structure of a piezoelectric thin film resonator disclosed in the Patent Publication. FIG. 28B is a cross sectional view taken along line A-A in FIG. 28A. In this case, an Si substrate that includes a flat principal surface is used as a substrate 41. A lower electrode 43 is composed of an Ru film (260 nm thick). A piezoelectric film 44 is composed of an AlN film (1200 nm thick). An upper electrode 45 is composed of an Ru film (260 nm thick).
An cavity 46 in the shape of a domed bulge is formed between the underside of the lower electrode 43 at a membrane region where the upper electrode 45 and the lower electrode 43 oppose each other, sandwiching the piezoelectric film 44, and a surface of the substrate 41. The shape of the cavity 46 is an ellipse, as illustrated in FIG. 28A. The shape of a part where the upper electrode 45 overlaps with the lower electrode 43 is formed so as to be substantially an ellipse. The cavity 46 can be formed by removing a pre-patterned sacrificial layer (not illustrated) under the lower electrode 43. Moreover, an etching solution inlet 47a for etching a sacrificial layer so as to form an cavity is provided in the substrate 41.
FIGS. 29A to 29C are cross sectional views for illustrating the process of fabricating the piezoelectric thin film resonator illustrated in FIGS. 28A and 28B. These drawings are cross sectional views taken along line A-A in FIG. 28A.
MgO (of the order of 20 to 100 nm thick) to be formed as a sacrificial layer film 50 is first deposited on the substrate 41 composed of Si (or a quartz substrate) by a sputtering technique or a vacuum evaporation technique, as illustrated in FIG. 29A. Then, the sacrificial layer 50 is patterned into a desired shape by a photolithography technique and etching.
Then, the lower electrode 43, the piezoelectric film 44, and the upper electrode 45 are formed in sequence, as illustrated in FIG. 29B. The lower electrode 43 is deposited by sputtering in an Ar gas atmosphere under a pressure of 0.6 to 1.2 Pa. The lower electrode 43 is further patterned into a desired shape by a photolithography technique and etching. Then, AlN to be formed as the piezoelectric film 44 is deposited by sputtering, using an Al target, in an Ar/N2 gas mixture atmosphere under a pressure of about 0.3 Pa. Then, an Ru film to be formed as the upper electrode 45 is deposited by sputtering in an Ar gas atmosphere under a pressure of 0.6 to 1.2 Pa. The upper electrode 45 and the piezoelectric film 44 are patterned into desired shapes by performing a photolithography technique and etching (wet etching or dry etching) on a piezoelectric thin film resonator deposited in this manner. In this case, the stress of the piezoelectric thin film resonator, which includes the lower electrode 43, the piezoelectric film 44, and the upper electrode 45, is set so as to be compressive stress. Moreover, a central part of an extraction part of the upper electrode 45 in contact with the membrane region is formed above an cavity 40 to be formed in the next step, and both ends of the extraction part of the upper electrode 45 are formed outside the cavity 40.
Then, an etching solution inlet 49 (refer to FIG. 28A) is formed in the lower electrode 43 by a photolithography technique based on resist patterning, as illustrated in FIG. 29C. The cavity 40 is formed by removing, by etching, the sacrificial layer 50 by injecting an etching solution from the etching solution inlet 49. In this case, the stress of the piezoelectric thin film resonator, which includes the lower electrode 43, the piezoelectric film 44, and the upper electrode 45, is set so as to be compressive stress. Thus, when this condition for the stress is satisfied, upon completion of etching of the sacrificial layer 50, the piezoelectric thin film resonator is inflated, so that the dome-shaped cavity 40 is formed between the lower electrode 43 and the substrate 41. The inlet for an etching solution for the sacrificial layer 50 may be provided in a central part of the membrane region, as disclosed in Japanese Laid-open Patent Publication No. 2007-208728.
While piezoelectric thin film resonators have the aforementioned features, as the size of devices that include piezoelectric thin film resonators has been reduced, the demand for a further reduction in the size of piezoelectric thin film resonators has arisen.
In general, piezoelectric thin film resonators are arranged in a two-dimensional array to constitute a filter, as in the case of surface acoustic wave devices. Thus, the size of a substrate that constitutes a filter is determined by the area occupied by piezoelectric thin film resonators on the substrate and the wiring. In order to reduce the size of a substrate, a method for reducing the area occupied by piezoelectric thin film resonators on a substrate surface by stacking the resonators may be considered. In Japanese Laid-open Patent Publication No. 2007-510383, a decoupled stacked bulk acoustic resonator band-pass filter is disclosed as such a stacked resonator. An upper film bulk acoustic resonator 120 is stacked on a lower film bulk acoustic resonator 110 with an acoustic decoupling material 130 therebetween, as illustrated in FIG. 5B in the Patent Publication.
This structure solves, by providing an acoustic decoupling material between upper and lower film bulk acoustic resonators, the problem of excessive coupling between the upper and lower film bulk acoustic resonators in a simply stacked thin-film bulk acoustic resonator (SBAR) illustrated in FIG. 3 in the Patent Publication.